1. Field of the Invention
The present invention relates to a motor or a polygon mirror drive motor which drives a polygon mirror for beam scanning, and, more particularly, to a motor provided with a magnetic fluid bearing.
2. Description of the Prior Art
In high speed motor such as a beam scanning polygon mirror motor of a laser beam printer, it has conventionally been necessary to support a rotary body by a bearing of small friction loss for the purpose of realizing high speed rotation with low vibration, and it has been customary to bear loads in the radial direction by a dynamic pressure pneumatic bearing and loads in the thrust direction by a magnetic bearing.
For example, Japanese Patent Unexamined Publication No. 61-157820 discloses a dynamic pressure radial bearing between a rotor and a frame body and, in order to quickly suppress vibration caused in the rotor due to disturbance, a closed air chamber and the open air are communicated with each other through a small hole.
In the above prior art, vibration due to disturbance is suppressed by forming the small hole in the closed air chamber so as to accelerate the damping of vibration or by preventing collision of metallic members by using a damper. However, this measure is mainly intended for the vibration in the thrust direction during the operation and the disturbance in the radial direction and contact of the radial bearing at the time of starting and stopping are not taken into consideration, resulting in a possibility that powder, produced by abrasion due to contact in the bearing portion, damages the mirror or seizure occurs in the bearing portion.
Further, in order to realize the high speed rotation with low vibration and less irregularity of rotation, it is essential to improve the dimensional accuracy of manufactured parts such as shaft and bearing individually, but this is not sufficient alone.
In other words, it has been often experienced from the standpoint of, engineer that, however, high the dimensional accuracy of each individual part may be improved, change of the ambient temperature or the like will adversely affect the combined accuracy of the individual parts, resulting in generation considerable vibrations.